Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce prime olefins such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in many processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, can be converted into prime olefins. This process is referred to as the oxygenate-to-olefin process. The preferred oxygenate for prime olefin production is methanol. The process of converting methanol-to-olefins is called the methanol-to-olefins process.
There are numerous technologies available for producing oxygenates, and particularly methanol, including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. The most common process for producing methanol is a two-step process of converting natural gas to synthesis gas. Then, synthesis gas is converted to methanol.
Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Synthesis gas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Synthesis gas is then processed into methanol. Specifically, the components of synthesis gas (i.e., hydrogen, carbon monoxide and/or carbon dioxide) are catalytically reacted in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one process, methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
The methanol is then converted to olefins in a methanol-to-olefins process and produces a reactor effluent stream. The reactor effluent stream contains desirable olefin product as well as byproducts. The byproducts are typically removed from any olefin product stream to make olefin product streams that are of an acceptable level of purity. Common byproducts in a methanol-to-olefin reactor effluent stream include several alkynes and/or dienes. Examples of alkynes and/or dienes include, but are not limited to, acetylene, methyl acetylene and propadiene. Alkynes and/or dienes are converted to olefins by a hydrogenation reaction. In a hydrogenation reaction, equimolar amounts of hydrogen and alkynes and/or dienes are reacted to produce a mole of olefin. The hydrogenation of olefin is a competing reaction whereby equimolar amounts of hydrogen and olefin are converted to a mole of paraffin. The reaction of alkyne to olefin is desirable because it eliminates an impurity, specifically the alkyne. The competing reaction is not desirable because it converts prime olefin product to less desirable paraffin byproduct.
Hydrogen, a reactant in the hydrogenation reaction, is often added to the alkyne containing olefin product stream to facilitate the reaction. Hydrogen is an impurity. Moreover, hydrogen often contains methane, CO and CO2. Likewise, these impurities can contaminate the olefin product stream, even when the hydrogenation reaction consumes all of the hydrogen. An added step of fractionation (typically a stripper) is required after hydrogenation to remove, excess hydrogen, methane, carbon monoxide and/or carbon dioxide from the olefin product stream after hydrogenation.
It would be desirable to have a hydrogenation step to eliminate alkynes and/or dienes that would result in an olefin product stream that has acceptable levels of alkynes and/or dienes, hydrogen, methane, carbon dioxide and/or carbon monoxide without the need for an additional fractionation step. It would likewise be desirable to hydrogenate as much of the alkynes and/or dienes as possible while hydrogenating as little olefin as possible. The present invention satisfies these and other needs.